Glass fibers are manufactured from various raw materials combined in specific proportions to yield a desired chemical composition. This collection of materials is commonly termed a “glass batch.” To form glass fibers, typically the glass batch is melted in a melter or melting apparatus, the molten glass is drawn into filaments through a bushing or orifice plate, and an aqueous sizing composition containing lubricants, coupling agents, and film-forming binder resins is applied to the filaments. After the sizing is applied, the fibers may be gathered into one or more strands and wound into a package or, alternatively, the fibers may be chopped while wet and collected. The collected chopped strands may then be dried and cured to form dry chopped fibers or they can be packaged in their wet condition as wet chopped fibers.
The composition of the glass batch and the glass manufactured from it are typically expressed in terms of percentages of the components, which are mainly expressed as oxides. SiO2, Al2O3, CaO, MgO, B2O3, Na2O, K2O, Fe2O3, and minor amounts of other compounds such as TiO2, Li2O, BaO, SrO, ZnO, ZrO2, P2O5, fluorine, and SO3 are common components of a glass batch. Numerous types of glasses may be produced from varying the amounts of these oxides, or eliminating some of the oxides in the glass batch. Examples of such glasses that may be produced include R-glass, E-glass, S-glass, A-glass, C-glass, and ECR-glass. The glass composition controls the forming and product properties of the glass. Other characteristics of glass compositions include the raw material cost and environmental impact.
Conventionally, lithium is added to glass fiber compositions in the form of spodumene (a lithium aluminosilicate raw material) to facilitate melting and obtain desirable mechanical and forming properties. For example, lithium is very effective in reducing the viscosity of the glass formulation. Although lithium-containing glass compositions may possess desirable properties with respect to mechanical and forming properties, the presence of lithium in the glass composition raises the cost of glass fiber manufacturing. This cost could be offset if the inclusion of lithium lowered the viscosity sufficiently to enable a high-performance glass such as R-glass to be melted in a refractory tank rather than in a platinum melter. R-glass is commonly melted in platinum melters.
There is a unique combination of forming properties that permit a glass to be melted and distributed in a conventional refractory tank and glass distribution system. First, the temperature at which the glass is held must be low enough so that it does not aggressively attack the refractory. An attack on a refractory can take place, for example, by exceeding the maximum use temperature of the refractory or by increasing the rate at which the glass corrodes and erodes the refractory to an unacceptably high level. Refractory corrosion rate is strongly increased as the glass becomes more fluid by a decrease in the glass viscosity. Therefore, in order for a glass to be melted in a refractory tank, the temperature of the refractory tank must be kept below a certain temperature and the viscosity (for example, resistance to flow) must be maintained above a certain viscosity. Also, the temperature of the glass in the melting unit, as well as throughout the entire distribution and fiberizing process, must be high enough to prevent crystallization of the glass (that is, it must be kept at a temperature above the liquidus temperature).
At the fiberizer, it is common to require a minimum temperature differential between the temperature selected for fiberizing (that is, forming temperature) and the liquidus temperature of the glass. This temperature differential, ΔT, is a measurement of how easily continuous fibers can be formed without production of the fibers being interrupted by breaks caused from devitrification. Accordingly, it is desirable to have as large a ΔT as possible to achieve continuous and uninterrupted glass fiber formation.
In the quest for glass fibers having a higher end performance, ΔT has, at times, been sacrificed to achieve desired end properties. The consequence of this sacrifice is a requirement that the glass be melted in a platinum or platinum-alloy lined furnace, either because the temperature exceeded the maximum end use temperature of the conventional refractory materials or because the viscosity of the glass was such that the temperature of the glass body could not be held above the liquidus temperature while producing a glass viscosity high enough to keep the refractory corrosion at an acceptable level. S-glass is a good example where both of these phenomena take place. The melting temperature of S-glass is too high for common refractory materials and the ΔT is very small (or negative), thus causing the glass to be very fluid and very corrosive to conventional refractories. Conventional R-glass also has a very small ΔT, and is therefore melted in platinum or platinum-alloy lined melters. The addition of lithium to the formulation sufficiently expands the ΔT of the R-glass to permit it to be melted in a standard refractory inciter. However, lithium raw materials are very expensive and greatly increase the manufacturing costs for the glass fibers.
Thus, there is a need in the art for high-performance, lithium-free glass compositions that retain favorable mechanical and physical properties (for example, specific modulus and tensile strength) and forming properties (for example, liquidus temperature and forming temperature) where the forming temperature is sufficiently low and the difference between the forming and liquidus temperatures is large enough to enable the components of the glass composition to be melted in a conventional refractory tank.